Process for nitriding



Patented June '2, 1936 UNITED STATES 7 2,042,527 rnooass FOR m'mmmeDonald Aubrey Holt, Niagara Falls, N. Y., assig'no'r to E. I. duPont deNemours & Company, Inc., Wilmington, Del., a corporation of Delaware NoDrawing.

Application November. 30, 1932, Serial No. 645,043 v 8 Claims. (01.148--15i This invention relates to a process for introducing nitrogeninto the surface layers of steel articles, and more particularly to theuse of fused salt baths for that purpose.

The art of nitriding steel articles consists in treating said articlesin contact with a nitrogenous substance, for instance ammonia, whereby ahard outer surface of iron nitride is produced. In commercial practicethis process is applied to 10 certain alloy steels at temperatures below600 C.

In one proposed method, the alloy steel is immersed in a fused salt bathcontaining substantial quantities of an alkali metal cyanide. Such acyanide bath, operated at relatively high temperatures, e. g. 800 to 9000., produces a mixed case; that is, it introduces both carbon andnitrogen into the steel. However, if the cyanidebath is used attemperatures below 600 0., nitriding predominates and only a smallamount of carbonis introduced.

The method of nitriding in most common use at the present time consistsin= heating the alloy steel in an atmosphere of ammonia. This processgives good results but is disadvantageous in that a relatively longperiodsof time is required for 'the nitriding action. The steel isusually heated in contact with the ammonia for a period of from- 15 to90 hours, in order to produce a satisfactory nitrided case. A furtherdisadvantage of ammonia nitriding is that relatively complicated andexpensive equipment is required, and-the vessel in which the nitridingoccurs or the walls thereof, must be replaced when the interior becomescoated with a layer of iron nitride. This nitride tends to catalyze thedecomposition of ammonia, thus causing waste and decreasing thenitriding effect. Proper control of the operation of the processrequires careful.

and constant attention. Moreover, the fact that ammonia must betransported in heavy, pres sure-resistant vessels, adds considerably tothe cost of this method.

Nitriding by means of low temperature treatment in fused cyanide bathsmaybe accomplished in a, shorter time than ammonia nitriding. However,the case produced is neither as hard nor as deep as that produced by thcammonia process. Moreover, there'is a certain tendency for carburizationto occur, with the result that in addi- 60 tionto superficialcarburization of the steel, a gray deposit, which may be either carbonor iron carbide, is deposited on the surface of the steel.

When the steel is subsequently polished, this re- An object of thisinvention is to provide a process for nltriding alloy steels which willgive results comparable with thoseobtainable by the ammonia process butin less time and at less expense. A"further object is to provide a fused5 salt nitriding bath which will not substantially carburize steel nordeposit carbon at nitriding temperatures, and will produce a clean,uniform, nitrided surface.

I have discovered that superior nitriding eflects 10 maybe obtained bytreating an alloy steel at a temperature below 600 in a bath of fusedsalt containing cyanamide. This method of -nitrlding produces nitridecases equal to those obtainable by the ammonia process and in a muchshorter 15 time. Furthermore, -I havefound that a fused salt bathcontaining a metal cyanamide, even in the presence of considerableamounts of cyanide, has substantially no carburizing action attemperatures below 600 C. In fact, under some con- 20 ditions such bathshave a .sllght decarburizing action. As a consequence, the casesproduced by this improved method are perfectly uniform and present acleamappearanoe when polished.

I prefer to use an alkali metal cyanamide, e. g. 25 sodium cyanamide, ina bath free from substantial amounts of alkaline earth metal compoundswhich might react with alkali metal cyanamide to form alkalineearthmetal cyanamides. The presence of substantial amounts of alkalineearth metal 39 cyanamides'fis' objectionable because these compoundshave relatively high melting points at the operating temperature, henceunduly increasing the viscosity of the bath. Moreover, as the alkalineearth metal cyanamides decompose during 35 the nitrlding process, theyform insoluble oxides and/or carbonates in contradistinction to thealkali metal cyanamides, which decompose to formsoluble oxides andcarbonates. The presence of insoluble salts or oxides in the bath 40causes uneven nitriding and causes overheating of the nitriding vesselbecause of the heat insulating layer that settles out on the bottom ofthe vesseL;

Theproportion of alkali metal cyanamide in 45 a fused salt bath capableof being used to carry out my invention, may vary within wide limits.

For example, abath containing around 1% of sodium cyanamide or even lessthan 1% will have a certain nitriding action. The process may also becarried out with large proportions of cyanamide; for instance, a bathoi'substantially pure sodium 'cyanamide may be used. However, the meltingpoint of'pure sodium cyanamide is'cl'ose to the maximum allowabletemperature for point of the mixture.

{ which are not incompatible with the cyanamide, may be added for thispurpose. Because of their cheapness and availability, I prefer to usefor this purpose sodium or potassium cyanide or a mixture of the two. Inplace of cyanides. other low-melting salts or salt mixtures may be used;for example, a mixture of lithium, sodium and potassium chlorides hasbeen found to be satisfactory. Whether cyanide or other salts are used,I preferto maintain the concentration of alkali metal cyanamide inthe'bath at 10-40% by weight.

Myprocess may be used to nitride any type of steel which has a surfacecomposition adapted for nitriding. By this, I mean a steel whosecomposition is such that nitrogen will be absorbed in substantialamounts when the steel is heated to a temperature of not more than 600C. in contact with'a nitrogenous substance, with the resuit that a hard,durable, nitrided surface is formed. Steels especially adapted fornitriding by my process are those which contain, either throughoutoronly in surface layers, one or more of the following alloyconstituents: aluminum, molybdenum, chromium, vanadium, manganese ors'ilicon. As in the case of other nitriding processes, the degree ofnitriding and the character of the case will depend to a large extentupon the amount and kind of alloy constituents present in the steel. i v

Obviously, by process may be used to increase the hardness of alloysteels, which have been partially hardened by other casehardenin'g ornitriding processes.

In general, my process may be carried out by immersing. the steel in afused salt bath containing a metal cyanamide at a temperature between400 to 600 C. for a period of time which will vary, depending upon thedepth of case and degree of hardness desired. In most cases it will notbe necessary to continue the treatment longer than 15 hours; forinstance,'by a 15 hour treat- 'ment, I have obtained nitrided caseshaving a depth of 0.012 inch, while cases 0.007 inch deep were obtainedin a 5 hour treatment, and as later described, increased hardness wasattained in certain steels with a one-half hour treatment The surfacehardness of such cases will be greater than 1000 Vickers-Brinell unitswith certain alloy steels. During thenitriding process the cyanamidecontent of the bath slowly decreases and must occasionally bereplenished. However, as briefly indicated above, the amount ofcyanamide in the bath is not important, providing there isaround one ormore per cent present at all times.

I have found that the rate of cyanamide decomposition may be greatlydecreased without impairing the nitridingeffect of the bath, bypartially excluding air 'from the surface of the molten bath by coveringthe surface with a floating layer 01' finely divided graphite orcharcoal. I prefer to use flaked graphite for this purpose, sincecharcoal has a certain tendency to react with the cyanamide, whereasgraphite is substantially inert. Partial exclusion of air may also beeffected by placing over the bath container and almost air-tight cover.

In one method of carrying out my invention, alloy steel is treated at400-600 C. in a fused salt 75 bath containing an alkali metal cyanamideand content of the bath remains substantially constant, and issubstantially independent of the amount of cyanamide present in thebath. Moreover, in the presence of the alkali metal cyanamide, thecyanide present in the bath has substantially no carburizing action uponthe steel, nor does it tend to depositout any appreciable amount ofcarbon on the steel, or elsewhere. One demonstration of this is the factthat during operation, the cyanide content of the bath re- 'ma.inssubstantially constant, except in case where a reactive form of carbon(e. g.- charcoal) is present. Furthermore, when my invention is carriedout with a fused salt bath containing no cyanide; for example,'a bathcomposed of sodium chloride, potassium chloride, lithium chloride, andsodium cyanamide, operated at temperatures below 600 C., the nitridingeffect is substantially the same as that obtained with baths containingcyanide and cyanamide.

My improved nitriding bath may be advantageously employed for drawing ortempering certain steels, e. g. high speed tool steels, which may betempered at temperatures below 600 C. In

such cases, the exterior portions of the steel will be increased inhardness because of the nitrid ing effect of the bath, while the steelis simultaneously tempered. The degree of surface hardness obtained bythis method will depend upon the time the steel remains in the bath; anappreciable hardening effect can be obtained in as short a time as 30minutes.

lowing examples:

Example I Percent Carbon 0.33 Manganese 0.51 Silicon 0.20 Aluminum 1.24Chromium 1.58 MolybdenunL 0.20

After removing the treated steel articles from the bath they were cooledin the air and tested for hardness by the Vickers-Brinell method. Thearticles were found to have a Vickers-Brinell hardness of 1200 units.The case depth obtained, as determined by grinding to expose the core,and etching with a solution of nitric acid in methanol, was found to be0.007 to 0.008 inch.

' Example II Samples of the alloy steel described in. Example I weretreated by inunersion in a fused bath composed of about equal parts byweight of potassium cyanide, sodium cyanide, and sodium cyanamide atabout 538 C. Samples treated in this bath for 5 hours were found to havea nitrided case 0.007 inch in depth and had a Vickers- Brinell hardnessof- 1100 units. Other samples, which remained in the bath for 15 hours,were My invention is further illustrated by the fol 35 found to have anitrided'case of 0.012 inch in depth and a Vickers-Brinell hardness of1200 units. Analyses of the bath during the operation showed that theaverage rate of decomposition of the cyanamide was about 3 per hour.

Example III 'sten were treated in the bath for various lengths of time.The average rate of decomposition of the sodium cyanamide during the runwas 0.28%

per hour. The results obtained are given in the following table:

Age of both T- I Vickers- Sample No. during Case depth Brinell treatmenttreatment hardness Hours Hours Inch 1 5 s I 0.007 1112 2 7-12 5 0.0071112 a 12-18 0 0.008 1200 4 l228 0. 014 1200 5 30-30 0 0.007 1112 0 l.3545 0 1200 7 55-01 0 0.004 1112 Samp1e No. 6 was the high 5 eed steel;

the other samples were the steel described in xample I. 7

At the end of the 61st hour the bath contained 5.8% of sodium cyanamide.

Example IV A mixture containing 37.6% by weight of lithium chloride,46.9% of potassium chloride, and 15.5% of sodium chloride was fused, andabout 10% by weight of sodium cyanamide was added to the melt. A sampleof the steel described in Example I was'immersed in the molten bath fora period of 2}/ hours at 530 C. After cooling, the sample had a hardnessof 1112 Vickers-Brlnell units. Another sample of this steel acquired aVickers-Brinell hardness of 1200 after a 4 hour treatment in this bathat 530 C. A sample of a chrome-vanadium steel, treated in the bath at530 C. for-4 hours, acquired a hardness of 734 Vickers-Brinell units.This steel had the following alloying constituents:

Per cent Carbon 0.30-0.40 Chromium 1.25

Vanadium 0.45

Example V and the hardness at various depths below the surface weredetermined in the case of a sample of the same steel which had beentreated for 8 hours in a bath consisting of an eutectic mixture ofsodium cyanide and potassium cyanide at 538 C. The following resultswere obtained:

Vickers-Briaell Hardness Depth below Steel treated steel t t d originalby bath of suflm Example In NaCN KON bath Inch Example VI -Approximatelyequal parts of sodium cyan- A sample of alloy steel having the followingconstituents also attained a hardness of 1000 Vickers-Brinell unitsafter a 4-hour treatment in this bath:

Percent Carbon--- 0.36 Manganese 0.51 Silicon 0.27 Aluminum 0123Chromium. 1.49 Molybdenum 0.18

A sample of a high speed steel of unlmown composition, which had beentreated one-half hour below 600 C. in a bath composed of approximatelyequal parts of sodium cyanide and potassium cyanide, had a surfacehardness of 950 Vickers-Brinell units. After a one-half. hour treatmentin the above described cyanamide bath, the hardness of this sample wasincreased to 1200 Vickers-Brinell units.

During a period of 5 hours, the average rate of decomposition of thesodium cyanamide in the above-described bath was 1.95% per hour. Samplesof the bath, removed at various intervals of time were in each casesubstantially white, showing that the charcoal did not penetrate intothe bath to any appreciable extent.

The rate of cyanamide decomposition in Example VI is higher than inExample III, probably because of the tendency of the charcoal to reactwith the cyanamide to form cyanide, whereas graphite does not so react.

My improved process for nitriding steel is capable of producing anitrided case of depth and hardness equal to that produced by theammonia process, and has the advantage that it is simpler in-operationand control, requires less expensive and less elaborate equipment, andrequires less expenditure for treating materials. A further advantage isthat it produces a greater degree oi. hardness and a deeper nitridedcase than is possible with nitriding baths heretofore used, and does notproduce an undesirable appearance in the nitrided article.

' ,I claim:

1. A process comprising heating a steel having a surface adapted fornitriding, in a fused salt bath comprising as the active nitriding agentan alkali metal cyanamide at a temperature below 600 C. while partiallyexcluding air from the surface of said bath.

2. A process comprising heating a steel having a surface adapted fornitriding, in a fused salt bath comprising as the active nitriding agentan alkali metal cyanamide at atemperature below 600 0., the surface ofsaid bath being covered with a layer of flake graphite.

3. A process comprising heating a steel having a surface adapted fornitriding, in a fused salt bath containing sodium cyanide and as theactive nitriding agent 1-40% by weight of sodium cyanamide atatemperature below 600 C. for 2 to 15 hours, while partiallyexcluding'air from the surface of said bath.

4. A process comprising heating a steel having a surface adapted fornitriding, in a fused salt bath containing sodium cyanide and as theactivenitriding agent 1-40% by weight of sodium cyanamide at atemperature below 600 C. for 2 to 15 hours, the surface of saidbathbeing covered with a layer of flake graphite.

5. A process comprising heatinga steel having a .surfaceiadapted fornitridin'g, in a fused salt bath containing 1-40% by weight of as theactive nitriding agent an alkali metal cyanamide' at a temperature below600 C., while providing means for allowing restricted amounts of air tocome in contact with the surface thereof.

6. A process comprising heating a steel having a surface adapted fornitriding, in a cyanide-free fused saltbath containing as the activenitriding agent-an alkali metal cyanamide at a temperature below 600 C.

7. A process comprising heating a steel having a surface adapted fornitriding in a cyanidefree fused salt bath comprising alkali metalchlorides and as the active nitriding agent 1 to 40% by'weight ofanalkali metal cyanamlde at a temperature below 600 C. I

8. A process comprising heating a steel having a surface adapted fornitrlding' in' a fused salt bath consisting of lithium chloride,potassium chloride, sodium chloride and 1 to 40% by weight of sodiumcyanamide at a. temperature below 600 C.

\ x DONALD AUBREY HOLT.

